Study of the surface resistance of superconducting niobium films at 1.5 GHz

A systematic study of superconducting properties of niobium films sputtered on the inner wall of radiofrequency cavities is presented. The measured qu antities include in particular the response to 1.5 GHz microwaves, the crit ical temperature, the penetration depth and the magnetic penetration field. In addition to films grown in different gas discharges (Xe, Kr, Ar and Ar/ Ne mixtures) and to films grown on substrates prepared under different cond itions, the study also includes bulk niobium cavities, The surface resistan ce is analysed in terms of its dependence on temperature, on RF field and, when relevant, on the density of trapped fluxons. A simple parameterisation is found to give a good fit to the data. Once allowance for the presence o f impurities and defects is made by means of a single parameter, the electr on mean free path, good agreement with BCS theory is observed. The fluxon-i nduced losses are studied in detail and their dependence on RF field, on te mperature and on the density of trapped fluxons is analysed. The residual r esistance is observed to be essentially uncorrelated with the other variabl es, suggesting that it is dominantly extragranular. In occasions very low r esidual resistances, in the n Omega range, have been maintained over a broa d range of RF field, indicating the absence of significant fundamental Limi tations specific to the film technology in practical applications such as t he production of accelerating cavities for particle accelerators. (C) 1999 Elsevier Science B.V. All rights reserved.